The invention concerns a quantum well optical device, in particular a device including quantum boxes or quantum wires applicable to optical and opto-electronic techniques, and methods of constructing this device.
In the field of long wavelength electromagnetic waves, and in particular wavelengths exceeding 2 .mu.m, few optical and opto-electronic devices have been made from semiconductors. The reason is that there are few energy levels which are not strongly coupled to excitations of the structure. The only exceptions are semiconductors having a narrow forbidden band.
We propose here a new family of optical and opto-electronic devices in which energy levels are determined freely by the dimension of quantum holes and lines constituted of semiconducting materials inserted in matrices of other semiconductors.
The quantification of the energy levels of electrons and holes in ultra-thin semiconductor films (.gtoreq.20 nm) is now well known (see the article "Fundamental Properties of III-V Semiconductor Two-Dimensional Quantized Structures: The Basis for Optical and Electronic Device Applications" by C. Weisbach in Semiconductors and Semimetals, Vol 24, R. Dingle Ed., Academic, New York 1987). This is shown in FIG. 1a in which an ultra-thin layer of semiconductor SC2 is placed between two semiconductors SC1 and SC3. The structure of the energy levels for the specially chosen semiconductors is shown in FIG. 1b; the quantified energy levels for the electrons (E.sub.1, E.sub.2) and the holes (E'.sub.1, E'.sub.2) in layer SC2 are also shown.
If we take the approximation of an infinitely deep potential well, the successive quantified energy levels are situated at a so-called confinement energy lying above the bottom of the conduction band and given by the formula: EQU E.sub.n =n.sup.2 .pi.2h.sup.2 /2m*L.sup.2
where
n is the order of the level,
h is the Planck constant,
m* is the effective mass of the particle (electron or hole),
L is the thickness of the film SC2.
We see that we can control freely the difference between the energy levels by the thickness of the film. This possibility is used in detectors where the range of sensitivity in wavelength is adjusted at the transistion E1-E2 by the film thickness.
If the thickness is sufficiently small there is only a single energy level within the well, the level E2 being situated above .DELTA.Ec in the continuum of the states of materials SC1 and SC3.
The extension to three dimensions of the concept of quantification of the energy levels is immediate. It is sufficient to consider a semiconductor box inserted in another semiconductor: the wave function of the electrons and holes will be quantified in the box if, as we explained in connection with FIG. 1b, the electron and hole energy levels are lower in the material constituting the box compared with the surrounding material (see FIG. 2).
What is unique about this system is that, in the infinite well approximation, the successive energy levels are given by the formula: EQU E.sub.nx,ny,nz =.pi..sup.2 h.sup.-2 /2m* (nx.sup.2 /Lx.sup.2 +ny.sup.2 /Ly.sup.2 +nz.sup.2 /Lz.sup.2)
where positive integers nx, ny and nz are the orders of the levels along the x, y and z axes,
Lx, Ly and Lz are the dimensions of the box along the x, y and z axes
The energy levels E.sub.nx,ny,nz can be separated by an energy which is either incommensurable with any phonon of the crystal, or greater than that of any phonon of the crystal. In this case, energy relaxation by transitions producing a single phonon in the crystal can not occur and the lifetime of the excited states becomes extremely long, of the order of the radiative lifetime. This situation is different from that in quantum films where the kinetic energy of the electrons along the film gives continuous energy levels of the type: EQU E.sub.nzk =nz.sup.2 .pi..sup.2 h.sup.2 /2m*Lz.sup.2 +h.sup.2 k.sup.2 /2m*
where k is the wave vector of the electrons describing their free transmission in the plane.
Owing to the continuous energy levels, there are always energy levels between which transitions can be induced by phonons of the structure, as shown in FIG. 3.
The invention makes use of this situation to allow the construction of optical devices including quantum wells of limited dimensions.